The present invention relates to an improved method of the flow-rate measurement control available for a POS (Point of Sales) (or refuel control system) of a gasoline service station. Conventionally, there are two methods of the flow rate control being applied to the refuel control system including; (1) method of directly controlling the motor of the refuel unit, and (2) method of controlling the flow rate. Of these, the latter method can improve accuracy of the refuelling operation. Conversely, this method involves complex configuration of the refuelling units, thus increasing cost.
To solve these problems, some of the prior arts had already proposed such techniques based on the former method that could simplify the overall construction and saves cost as well. Typically, one prior art operation based on the former method had proposed such a system as described below. For example, when installing a new refuelling unit, a specific volume (about 7 liters) of gasoline is discharged to preliminarily determine the actual difference of the flow rate to be made available for the reference of the flow rate difference (.alpha.) caused by the inertia of the pump motor so that this data can be preset in memory of the control unit. In the actual refuelling operation, the system causes the pump motor to stop upon arrival at the value produced by subtraction of the value (.alpha.) from the designated refuel volume Q, thus properly controlling the refuel operation. Nevertheless, even after introducing such a new control method wherein a predetermined difference value (.alpha.) is applied to the refueling operation a significant difference will still occur since the refuelling units themselves were subjected to the degradation of performance characteristics and varying environmental conditions. When refuelling a small amount of gasoline, for example, less than 7 liters, the difference actually tends to increase furthermore. In the light of such a problem, a special refuel control system has been proposed in the Japanese Patent Laid-Open No. 204895 of 1982, which is aimed at a high-precision refuel control which minimizes the difference of the control value. The refuel control system of this Japanese Patent Laid-Open document was designed to securely minimize the difference value (.alpha.) in its practical applications by considering possible degradation of the mechanical characteristics and variable environmental conditions. More particularly, by presetting the mean value of the difference (.alpha.) detected during the previously refuelling operations covering the last several rounds, the refuel control system automatically corrects the difference value (.alpha.) so that the actual difference value can be obtained in response to those varying conditions. However, it was eventually found that even when applying such a control system, it was very hard to precisely correct the difference generated during the initial stage of the refuelling operation. In other words, when the pump motor had been turned on at the start of the refuel operation and while no gasoline was actually fed through the nozzle, a number of unnecessary flow-rate measuring pulses were generated by the flow-rate pulse generator, due to the varied pressure inside the refuel pipe. As a result, an unexpected difference value suddenly appeared during the refuelling operation.